The type of coating powder to which this invention is directed contains as the primary resin a polymer having an acrylic backbone and pendant epoxy functionality. Such polymers are formed, for example, from glycidyl esters of acrylic acids, and generally, additional unsaturated co-monomers. The coating powder also contains an epoxy-reactive curative, generally a crystalline multi-functional carboxylic acid, such as dodecanedioic acid. The coating powders may be clear, i.e., unfilled, or may contain fillers and pigments.
While the coatings formed from these coating powders exhibit high gloss and weatherability, a general disadvantage of the coatings is that they exhibit poor flexibility and impact resistance. It is a general object of the present invention to improve the flexibility and impact resistance of coatings formed from epoxy-functional acrylic coating powders.
U.S. Pat. Nos. 3,817,946 and 3,857,905, the teachings of which are each incorporated herein by reference, describe coating compositions containing an epoxy functional acrylate, an acid-functional polyester and a dicarboxylic acid. The compositions of this type described in these patents would not, however, have significant commercial utility as the polyester resins used therein have low melt onset temperatures, i.e., about 31.degree.-32.degree. C., which would result in sintering of the coating powder (agglomeration of particles) during typical storage conditions, rendering the powder unfit for coating purposes.